All commercially available in-die tapping (IDT) systems employ some sort of spring-loaded misfeed-protection system, intended to protect the tap and, most importantly, the tapping device itself, in the event of a:

• No-hole condition due to a short feed of the strip material;

• No-hole condition due to a broken or missing pierce punch; or

• Undersized hole due to a broken or missing extrusion punch.

Although misfeed-protection systems are included in commercial IDT designs as failsafe, protective mechanisms, it’s wise for metalformers to employ some method to ensure the successful production of a pretapped hole with every press cycle.

Sensor Basics

The types of sensors appropriate for use with IDT are those capable of detecting the physical presence (or absence) of an object. These sensors are generally configured for use with low-voltage (3-24 V DC) logic circuits. The most suitable types:

• Mechanical contact switches—the least costly option, but generally may have a shorter life expectancy than noncontact alternatives.

• Photo-optic sensors—work on the principle of an interrupted light beam, and are available in several configurations:

1) Separate emitter and receiver;

2) Retroreflective—Emitter and receiver are contained within a single assembly, with a separate mirror to reflect the beam;

3) Diffuse-reflective—Emitter and receiver are contained within a single assembly and the object to be detected reflects the light beam.

Fig. 1 and 2—The most rudimentary sensor configuration used in conjunction with IDT, a hot-pilot system checks for the actual presence of a pretapped hole before it reaches the IDT station. Stampers typically locate this assembly within the lower die, with the pilot punch pointing upward.

Note: The use of photo-optic sensors in IDT applications may become complicated if the light beam is exposed to tapping-lube mist.

• Inductive proximity sensors—These operate on the principle of eddy-current disturbances within the sensor’s electromagnet coil when an object is within range. Due to their simplicity, lack of any moving parts and (when Teflon-coated) indifference to fluid exposure, these sensors are the type most commonly used in stamping dies.

Regardless of the sensor type used, the primary objective is to send a logical True/False signal to a programmable logic controller (PLC) to confirm a desirable condition or annunciate an undesirable condition, and subsequently trigger an E-stop of the stamping press, as required. Sensor circuits are configured either as normally open (NO) or normally closed (NC), depending on the condition to be detected.

Prehole Detection

The most rudimentary sensor configuration used in conjunction with IDT simply checks for the actual presence of a pretapped hole before it reaches the IDT station. Sometimes referred to as a hot-pilot system, this configuration typically comprises a switch, a precision-guided floating pilot punch, a lever or plunger, and a compression spring (Fig. 1 and 2). Stampers typically locate this assembly within the lower die, with the pilot punch pointing upward. However, such an arrangement requires a certain amount of strip lift. For applications having minimal or no strip lift, a stamper can mount the system within the stripper pad with the pilot pointing downward, provided the signal leads from the sensor can be reliably managed with the stripper in motion. Note: Radio-frequency-transmitting sensors—increasing in popularity and decreasing in cost—are particularly suited for applications where dealing with signal leads/wiring is undesirable.

Yet one more option for applications with no strip lift: Use a shielded inductive-proximity sensor (configured to ignore any surrounding metal) at or just below die level to sense the presence of the prehole after the strip has fed into progression.

Fig. 3—Once the misfeed spring force equals the tap’s required axial engagement force, the tap bites into the part (right) and tapping begins, albeit late. As a result, valuable rotational movement required for tapping has been expended, resulting in an incompletely tapped thread. The left image illustrates the tap’s approach, while the center image illustrates successful tap penetration.

Tap Penetration and Wear

One of the requirements for achieving a quality thread is driving the tap to its proper depth within the workpiece. Typical nonbottoming taps for IDT will have 2.5 to 5 lead-in threads, and to achieve a complete thread, the tap must drive through the workpiece beyond all of its lead-in threads. If the tap is set too far from the workpiece, it will likely fall short of successfully penetrating the part by the end of the tapping stroke. And, if the tap is set too close to the workpiece, another set of problems (which are beyond the scope of this article) can occur.

Aside from incorrect tap-setup position, there’s another scenario that also can result in the tap failing to fully penetrate the workpiece—tap wear. The IDT misfeed system’s compression spring must supply sufficient preload force to cause the tap to immediately bite into the prehole upon contact. Rollforming/cold-forming taps predominantly are used in IDT applications and require much more axial force (to engage the prehole) than does an equivalent cutting-type tap. The natural tendency of the first thread of a rollforming tap is to skid against the prehole’s opening–particularly in the presence of a chamfer or rolled edge. This tendency increases as the tap wears.

Thus, the axial force required for tap engagement also increases over time, which can sometimes present a confusing problem to diagnose. When the tap is new and sharp, parts are produced with threads fully tapped to the required depth. However, as the tap wears it can start to skid if the required axial engagement force exceeds the available preload force stored in the misfeed spring. As the tapping unit turns the tap, the female lead bushing retracts inward and the misfeed spring undergoes further deflection, because the tap is rotating but not entering the prehole.

To Better Envision
this Scenario

…consider how a jack screw operates. As the misfeed spring further compresses, the force it exerts against the tap increases. Once the spring force equals the tap’s required axial engagement force, the tap bites into the part and tapping begins, albeit late. As a result, valuable rotational movement required for tapping has been expended in compressing the misfeed spring upon entry, resulting in an incompletely tapped thread (Fig. 3).

In the pressroom, this condition can easily be recognized when performing a QC check and the thread gauge nearly makes it through the part cleanly but goes tight within the last turn or two. Note: Most IDT suppliers add some safety margin (generally an extra half-revolution, or half of the tap’s pitch) to insure against this condition. However, it’s worthwhile to recognize that the male and female lead bushing of the pitch insert (tap cartridge) will undergo accelerated wear whenever partial misfeed spring actuation occurs.

To combat the problem of incomplete-depth threads, use a sensor to look for the tap at the end of its stroke (Fig. 4). Since mechanically driven IDT units als reverse direction at press bottom-dead center, stampers must configure the PLC of their press controls or die-protection center to read the sensor output when the press ram is on bottom. For example, “look on” or “ready” at 175 deg., and “look off” at 185 deg.

For optimum results, the misfeed spring must remain in a purely static condition throughout the entire tapping cycle. Stainless steel and HSLA typically are more resistant to initial tap engagement and require greater preload in the misfeed spring. Undersized prehole diameters also increase the tap engagement challenge, and, as a reminder, attempting to tap more than 75 percent of theoretical full thread is never recommended.

To combat the problem of incomplete-depth threads, use a sensor to look for the tap at the end of its stroke (Fig. 4). Since mechanically driven IDT units als reverse direction at press bottom-dead center, stampers must configure the PLC of their press controls or die-protection center to read the sensor output when the press ram is on bottom. For example, “look on” or “ready” at 175 deg., and “look off” at 185 deg.

Sensor Locations

Any idle progression before the IDT station presents an ideal location for a hot-pilot arrangement to detect the presence of a simple pierced or extruded prehole. With dies that lack idle stations, stampers can install a hot-pilot system that shares another progression that may be performing other operations.

Detecting successful tap penetration of course must be done in the tapping station above or below the tapped hole, depending on tapping-device orientation.

Note: There’s an astounding amount of comprehensive sensor know-how available in the marketplace, so this article intends only to motivate die designers and stampers to consider the use of sensors for IDT applications.